Fibrillar Adhesives in Nature
Nature provides endless inspiration for solutions to engineering challenges. Particularly at the small (sub-millimeter) scale, millions of years of evolution has resulted in fascinating structures with unique, sometimes non-intuitive properties. In the case of small agile climbing animals, fibrillar foot-pads as a solution for gripping surfaces has evolved many times. Similar structures are present in animals of different phyla, including arthropods (spiders, insects), and chordates (lizards), suggesting independent evolution. There is also evidence that these structures evolved independently within different types of lizards (Geckos, Anoles, and Skinks), with slightly different resulting structures [D. Irschick, A. Herrel, and B. Vanhooydonck, “Whole-organism studies of adhesion in pad-bearing lizards: creative evolutionary solutions to functional problems,” Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology, vol. 192, no. 11, pp. 1169-1177, 2006].
There exist a wide variety of fibrillar adhesives across the wide variety of animals, which utilize these structures. Some insects have fibrillar foot pads which secrete oily fluids which aid in adhesion, while others have completely dry structures. Adhesive pads which do not utilize secretions are called “dry adhesives,” as they leave no residue on the surfaces to which they adhere. Dry adhesives exhibit many unique adhesive properties. They act similar to a pressure sensitive adhesive such as tape, but are highly repeatable with long lifetimes, do not require cleaning, and, often in combination with small claws, adhere to surfaces which are anywhere from atomically smooth silicon to extremely textured rock. Furthermore, they exhibit directional properties, adhering in one direction, and easily releasing from the surface when loathed in another. Adhesion pressures as high as 200 kPa have been demonstrated for gecko subdigital toepads and single fiber (seta) measurements exhibited adhesion pressures greater than 500 kPa (50N/cm2) [K. Autumn, “Biological Adhesives,” Springer Berlin Heidelberg, 2006]. Using advanced fibrillar adhesives, several gecko species are capable of carrying up to 250% of their own body weight up a smooth vertical surface. Dry fibrillar adhesives are also quite power efficient. They can be attached and detached from surfaces with very low forces by means of special loading and peeling motions. Once adhered to a surface, they require no power to maintain contact, and resist detachment for long periods of time.
Interestingly, and against intuition, the material that makes lip these high performance adhesive footpads is not sticky at all. The fibers are made from a β-keratin, much like bird claws and feathers. It is the small size-scale and geometrical structure, which allows this material to act as a powerful and versatile attachment mechanism.
Mechanics of Fibrillar Adhesion
The hair-like structures of gecko footpads have fascinated scientists for well over a century, with various hypotheses about the mode of attachment. In 1884, Simmermacher proposed the hypothesis that gecko lizards might adhere to surfaces using micro-suction cups [G. Simmermacher, “Untersuchungen ber haftapparate an tarsalgliedem von insekten,” Zeitschr. Wiss. Zool, vol. 40, pp. 481-556, 1884]. Fifty years later, Dellit carried out experiments in a vacuum winch demonstrated that suction is not the dominant attachment mechanism in geckos [W. D. Dellit, “Zur anatomie and physiologie der geckozehe,” Jena. Z. Naturw, vol. 68, pp. 613-656, 1934]. Similarly, electrostatic adhesion and micro-interlocking were ruled out. It was not until the advent of the Scanning Electron Microscope that scientists were able to investigate the true structure of these microscopic features. What they observed is a forest of microscale fibers, each branching into finer and finer hairs, ending in spatula-like tips. It is this structure that turns the stiff keratin into a capable adhesive.
Conventional pressure sensitive adhesives such as adhesive tapes, gels, and soft elastomers function by deforming into the shape of the contacting surface when pressed into contact. Materials with very low Young's modulus (stiffness) conform to surfaces to create large contact areas and do not store enough elastic energy to induce separation from the surface after the loading is removed. However, due to their low modulus, these materials tend to pick up contaminants from the surface, and are typically not re-usable.
Stiffer materials do not easily conform to surface roughness, and if deformed into intimate contact through high loading, store enough elastic energy to return to their original shape, peeling away from the surface in the process of relaxation. Bulk stiff materials generally do not exhibit tackiness or adhesion due to this self-peeling behavior.
The structures found in the attachment pads of the animals described above consist of arrays of thousands or millions of hair-like fibers, which stanch vertically or at an angle from the pad surface. Each fiber acts independently and generally has a specialized tip structure. The hairs in these fibrillar adhesives conform to the roughness of the climbing surface to increase the real contact area much like the deformation of soft adhesive tape, resulting in high adhesion by surface forces [K. Autumn et al., “Adhesive force of a single gecko foot-hair,” Nature, vol. 405, pp. 681-685, 2000]. This adhesion, called dry adhesion, is argued to arise from molecular surface forces such as van der Waals forces [K. Autumn et al., “Evidence for van der Waals adhesion in gecko setae,” Proceedings of the National Academy of Sciences USA, vol. 99, pp. 12252-12256, 2002], possibly in combination with capillary forces [G. Huber et al., “Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements,” Proceedings of the National Academy of Sciences USA, vol. 102, pp. 16293-16296, 2005]. Although the total potential contact area of a surface broken up into fibers is less than the area of a flat surface because of the gaps between the fibers, the ability for each fiber to bend and conform to the surface roughness allows thousands, millions or billions of fibers no make small individual contacts, which add up to a large surface area. In comparison, a flat surface only makes contact with the asperities of a surface, and since the deformations of bulk material are typically small, the total contact area is much less than in the fibrillar case. An illustration of this comparison can be seen in FIGS. 1a and 1b, which illustrate the contact area of a flat stiff material 2 against a rough surface 4 (FIG. 1a) can be less than the contact area of a fibrillar adhesive 6 against the same surface 4 (FIG. 1b) despite the area lost between the fibers.
Because of the high aspect ratio (height to diameter) of the fibers in FIG. 1b, the fibrillar surface's effective modulus is low despite the material modulus typically being quite high. The keratinous materials found in geckos' fibers are estimated to have a Youngs modulus of approximately 1-2.5 GPa. However, due to their hairy structure, the effective modulus is closer to 100 kPa, much like a soft tacky elastomer.
Animals with very low mass, such as insects, generally have a simply micro-fiber structure with specialized tips. In large lizards such as the Tokay gecko the fibers take on a complicated branched structure with microscale (4-5 μm) diameter base fibers which branch down to sub-micron (200 nm) diameter terminal fibers. At the end of these terminal fibers are specialized tips.
The most advanced fibrillar dry adhesives are found in the heaviest animals such as the Tokay and New Caledonia Giant Gecko gecko which can weigh up to 300 grams. Gecko toes have been shown to adhere with high interfacial shear strength to smooth surfaces (88-200 kPa). These animals have adhesive pads with many levels of compliance including their toes, foot tissue, lamellae, and fibers. This multi-level hierarchy allows the adhesives pads to conform to surface roughness with various frequency and wavelength scales. The fibers are angled with respect to the animals' toes, and the branched tips are also oriented with respect to the base of the fiber. The result is that the gecko pad exhibits a high level of directional dependence, high adhesion while dragging the toe inwards, and no adhesion in the opposite direction. This directionality is sometimes referred to as frictional anisotropy or, more appropriately, directional adhesion.
Studies of gecko footpads have revealed that due to their asymmetric angled structure, they are non-adhesive in their resting state, and a dragging motion is required to induce adhesive behavior [K. Autumn et al., “Frictional adhesion: a new angle on gecko attachment,” Journal of Experimental Biology, vol. 209, pp. 3569-3579, 2006]. Reversing the direction of this dragging motion removes the fibers from the surface with very little force.
Motivation for Fabrication of Dry Adhesives
The dry fibrillar adhesive structures found in nature exhibit properties, which may be highly desirable in synthetic materials. The mechanics which gives rise to the adhesion in these structures does not rely on liquids or pressure differentials, therefore fibrillar dry adhesives are uniquely suited for a variety of uses. Since dry adhesives leave no residue and can grip over large areas, they could be used as grippers for delicate parts for transfer and assembly of anything from computer chips in a clean-room to very large porous carbon-fiber panels for vehicle construction.
If manufactured inexpensively, synthetic dry adhesives could also find uses in daily life as a general adhesive tape for hanging items, fastening clothing, or as a grip enhancement in athletic activities such as gloves, shoes, and grips.
Man-made dry adhesives might be used for temporary attachment of structures during assembly, or allow astronauts to grip the smooth outer surfaces of spacecraft during extravehicular missions.
Since biological dry adhesives allow animals to climb on smooth surfaces, synthetic dry adhesives should enable robots to do the same. Robots with dry adhesive grippers may be used for inspection and repair of spacecraft hulls, or terrestrial structures. Since the adhesives require no power to remain attached, climbing robots could perch for days, weeks, or months with very little power usage. Also, due to the power efficient attachment and detachment, robots might move as easily up a wall as they currently traverse the ground. Similarly, one day, gloves covered in synthetic dry adhesives might allow humans to easily scale smooth vertical surfaces.
There are potential applications for fibrillar adhesives in the field of medicine as well. Safe, non-destructive temporary tissue adhesives could assist in surgical procedures. Capsule endoscopes might use fibrillar adhesives to anchor to intestine walls without damaging the tissue in order to closely examine or biopsy an area of interest. Fibrillar adhesives may also be designed for attachment to skin as an alternative to conventional adhesive bandages and patches.
Prior Art
Synthetic Fibrillar Adhesives
In 2000, when Autumn et al. published work measuring the adhesion of a single gecko seta, suggesting that it is the van der Waals intermolecular forces dominantly, which allow geckos to climb, it spawned a field of research into understanding and modeling the underlying principles of fibrillar adhesion, and fabricating synthetic mimics. Soon after, Autumn et al. demonstrated van der Waals forces and a unique geometry are primarily responsible for the adhesion. Sitti and Fearing created the first synthetic fibrillar adhesives by silicone rubber micromolding in the same year [M. Sitti and R. S. Fearing, “Nanomolding based fabrication of synthetic gecko foot-hairs,” In Proceedings of the IEEE Nanotechnology Conference, pp. 137-140, 2002].
In the years since then, there has been a flurry of research, with more than 50 publications on the topic in 2007 alone. Autumn continues to test biological specimens which provide insights into the mechanisms of adhesion, self-cleaning [W. R. Hansen and K. Autumn, “Evidence for self-cleaning in gecko setae,” Proceedings of the National Academy of Sciences USA, vol. 102, no. 2, pp. 385-389, 2005], and the directional properties of real gecko setae.
Huber and Sun demonstrated evidence that suggests that capillary forces of ambient water layers on surfaces play a significant role in fibrillar adhesion. Contact mechanics researchers such as Persson, Crosby, and Hui have investigated the crack trapping nature of patterned and fibrillar surfaces, which they have shown to increase the adhesion and toughness of the interfaces. In addition, Hui studied the bending and buckling nature of fibrillar surfaces, and the effects of this behavior on the adhesion of simple pillars. Arzt has investigated the effects of scale and shape of natural fibrillar adhesives, concluding that tip shape has less importance at smaller size scales. Several groups have demonstrated an inverse correlation between animal size and fibril tip dimension, with the heaviest animals having the finest fiber structures [E. Arzt, S. Gorb, and R. Spolenak, “From micro to nano contacts in biological attachment devices,” Proceedings of the National Academy of Sciences USA, vol. 100, no. 19, pp. 10603-10606, 2003].
The mechanics of fiber to fiber interactions have been studied and modeled to determine the proper spacing and patterning for a high density of fibers without clumping. Fibers will clump together if the adhesion energy between neighboring fibers is greater than the stored elastic energy of the fibers-bending into contact. The resulting equations can be used to calculate the closest spacing without permanent collapse.
The effects of crack trapping on increasing the toughness and adhesion of fibrillar surfaces have been studied on the macro-scale as well as the micro-scale. Several structures have been tested, and show enhancement over non-fibrillated structures.
The roughness adaptation of gecko pads has also been investigated through testing and modeling. The mechanics of fiber deformation and buckling reveals that fibrillar structures can decrease the effective modulus of the surface by several orders of magnitude, allowing conformation to various rough and curved surfaces.
In addition to research to understand and model the mechanics of adhesion, several research groups have developed fabrication techniques to create synthetic fibrillar arrays. Since van der Waal's forces are universal, a wide variety of materials and techniques may be used to construct the fibers. Initially, simple vertical fiber arrays were fabricated from various materials such as polymers. Methods such as electron-beam lithography, micro/nanomolding, nanodrawing, and self-assembly are employed to fabricate fibers from polymers, polymer organorods, and multi-walled carbon nanotubes.
Generally, arrays of simple pillar structures were not effective in increasing the adhesion of surfaces. Significant adhesion enhancement was demonstrated only when the flat tips of the structures were fabricated to have higher radii for increased contact area. Gorb et al. fabricated polyvinylsiloxane fibers with thin plate flat mushroom tips which demonstrated adhesion enhancement as well as contamination resistance [S. Gorb et al., “Biomimetic mushroom-shaped fibrillar adhesive microstructure,” Journal of The Royal Society Interface, vol. 4, pp. 271-275, 2007]. Similarly, Del Campo et al. developed techniques for forming flat mushroom tips as well as more complex 3D geometries, including asymmetric tips, by dipping [A. Del Campo et al., “Patterned surfaces with pillars with controlled and 3d tip geometry mimicking bioattachment devices,” Advanced Materials, vol. 19, pp. 1973-1977, 2007]. Kim et al. developed fabrication methods to form microscale fibers with flat mushroom tips by exploiting the champagne glass effect during Deep Reactive Ion Etching to form negative templates in silicon on oxide wafers [S. Kim and M. Sitti, “Biologically inspired polymer microfibers with spatulate tips as repeatable fibrillar adhesives,” Applied Physics Letters, vol. 89, no. 26, pp. 261911, 2006]. In addition, Kim demonstrated the importance of controlling the thickness of the backing layer in order to reduce coupling between fibers.
Glassmaker et al. fabricated polymer fibers topped with a terminal film which exhibited adhesion enhancement over tipless pillars and unstructured surfaces [Nicholas J. Glassmaker et al., “Biologically inspired crack trapping for enhanced adhesion,” Proceedings of the National Academy of Sciences, vol. 104, pp. 10786-10791, 2007]. Angled pillars with a terminal film have also been fabricated with directional properties [H. Yao et al., “Adhesion and sliding response of a biologically inspired fibrillar surface: experimental observations,” Journal of The Royal Society Interface, vol. 5 no. 24, pp. 723-733 2007]. By angling the pillars beneath the terminal film, the resultant structure exhibits anisotropic adhesion. In addition to stern angle, the angle of the surface of the tip with respect to the stem is as crucial in terms of controlling the anisotropic behavior in adhesion and friction. Kim et al. [S. Kim et al., “Smooth Vertical Surface Climbing With Directional Adhesion,” IEEE Transactions on Robotics, vol. 24, no. 1, pp. 1-10, 2008] fabricated synthetic sub-millimeter wedges with the stern and tip surface of each individual wedge oriented at an angle with respect to the backing layer of the wedge array. These structures exhibited anisotropic friction much-like the biological counterparts. While the magnitude of friction was an order of magnitude less than the biological gecko footpads, adhesion in normal direction was negligible. Later Asbeck et al. [A. Asbeck et al., “Climbing rough vertical surfaces with hierarchical directional adhesion,” IEEE International Conference on Robotics and Automation, Kobe, Japan, 2009] fabricated similarly shaped wedges that are an order of magnitude smaller which showed similar adhesion performance to the sub-millimeter wedges. Adhesion improvement, still low compared to the biological gecko footpad, occurred when they topped sub-millimeter wedges with a terminal film comprised of micro-wedges.
Higher modulus synthetic fibrillar adhesives have been developed on the sub-micron diameter scale. These fibers, made from stiffer materials (E>1 GPa) such as polypropylene, polyimide, and nickel, carbon nanofibers and carbon nanotubes. Although these stiffer fibers do not adhere well in the normal direction, and require high preloads to make intimate contact, shear adhesion pressures of up to 36 N/cm2, which is higher than the adhesion strength of the gecko, have been demonstrated.
To more closely mimic the structure of the gecko's foot hairs, work has also been done to fabricate hierarchical fibers with multi-level structures. Ge et al. bundled carbon nanotubes into pillars which deform together but have individually exposed tips. [L. Ge et al., “Carbon nanotube-based synthetic gecko tapes,” Proceedings of the National Academy of Sciences, vol. 104, no. 26, pp. 10792-10795, 2007]. Photolithography has been used to fabricate simple micro-pillars on top of base pillars [A. Del Campo and E. Arzt, “Design parameters and current fabrication approaches for developing bioinspired dry adhesives,” Macromolecular Bioscience, vol. 7, no. 2, pp. 118-127, 2007]. On the millimeter scale, Shape Deposition Manufacturing has been used to fabricate hierarchical structures in thin polymer plates, which are stacked into arrays [M. Lanzetta and M. R. Cutkosky, “Shape deposition manufacturing of biologically inspired hierarchical microstructures,” CIRP Annals—Manufacturing Technology, vol. 57, pp. 231-234, 2008]. Kustandi et al. demonstrated a fabrication technique to use nanomolding in combination with micromolding to create a hierarchical structure with superhydrophobic properties.
In addition to dry adhesives, other work is being conducted on synthetic fibers with oily coatings, inspired by beetle adhesion, which exhibit increased adhesion over uncoated structures.
The microfiber fabrication methods described above are very expensive for producing commercial quantities of adhesive materials. Moreover, they cannot efficiently and controllably produce angled fibers with specialized tips or hierarchical structures with specialized tips. Accordingly, there is a need for improved dry adhesives and improved methods for making dry adhesives. In particular, there is a need for dry adhesives having greater adhesive forces and improved durability. In addition, there is a need for methods of making dry adhesives with lower costs of production. Those and other advantages of the present invention will be described in more detail hereinbelow.